Bulgarian Foreign Minister Nikolay Mladenov said it was an extremely intensive investigation.

"The results of that investigation leads to a number of persons who are connected to the military wing of Hezbollah," he said.

The suspects include two people who carried Canadian and Australian passports. Mladenov told BBC World that this is something the EU needs to consider seriously.

"We should discuss all measures that we need to do collectively to make sure that we can protect ourselves from terrorist attacks in the future," he said.

The White House picked up that same line, saying Bulgaria's investigation shows that Hezbollah is a "real and growing threat not only to Europe, but to the rest of the world."

Secretary of State John Kerry condemned Hezbollah and said the U.S. was working to curtail the group's activities.

"We strongly urge other governments around the world and particularly our partners in Europe to take immediate action to crack down" on Hezbollah, he said in a statement. "We need to send an unequivocal message to this terrorist group that it can no longer engage in despicable actions with impunity."

Matthew Levitt, a former Treasury Department official who has dealt with these issues, says it would be significant if Europe agrees to designate Hezbollah as a terrorist organization.

"Right now, Hezbollah is able to raise funds in most European countries overtly, like the Red Cross," said Levitt, who now runs the counterterrorism program at the Washington Institute for Near East Policy. "And there's nothing illegal about it."

Some countries make a distinction between the military and political wings of Hezbollah, which is part of the government in Lebanon. France worries that putting Hezbollah on a terrorism list could further destabilize Lebanon, but Levitt warns that Hezbollah's activities are on the rise, and European law enforcement officials need new tools to go after the group.

"The attack that succeeded was not the first time Hezbollah tried to do this, and not the first time it tried to do it in Bulgaria," Levitt said. "Six months earlier, a Hezbollah squad trying to target Israeli tourists on their way to a ski resort in Bulgaria were thwarted; and just a week before this attack in Bulgaria last July, another Hezbollah operative was arrested in Cyprus, where he was conducting the same kind of surveillance."

Levitt, who has written a soon-to-be-published book tracing Hezbollah's activities around the world, says U.S. counterterrorism officials are increasingly worried about the Lebanese militant group and its use by Iran as a proxy.

"There's a lot going on right now, and when I talk to U.S. officials, they tell me that on some days the No. 1, 2 or 3 and sometimes all three of the top things being briefed to senior officials are not only or just Sunni al-Qaida-type jihad anymore. It's Iran ... and Hezbollah."

Israeli Prime Minister Benjamin Netanyahu has also warned that Iran and Hezbollah are building, as he puts it, a worldwide terrorist network.